This invention relates to a method for fabricating filamentreinforced composite articles. In particular, this invention is directed to a method for fabricating filament-reinforced titanium aluminide matrix composite materials.
Composite metallic structures, which are reinforced with high strength, high modulus filaments or fibers having a high length-to-diameter ratio, have been demonstarted to have high specific properties. With particular respect to the aerospace industry, titanium-based composites have been considered for high temperature applications because of the high-temperature strength and low density of titanium and its alloys. Fiber-reinforced titanium-based composites exhibit increased temperature capability, improved shear, transverse, and off-axis properties; and better erosive environment durability when compared with presently available aluminum matrix and polymeric matrix composite systems.
Previous attempts to fabricate fiber-reinforced titanium alloy matrix composite materials have met with only limited success. In order to provide a usable product, sheets of the matrix material and layers of the reinforcing fibers are stacked so that the top of each reinforcing fiber is positioned opposite the bottom of a superimposed metal sheet. The stacked layers are laminated, typically by a vacuum hot pressing operation, into an integrally bonded composite structure. It has been established that, at consolidation temperatures sufficiently high to promote bonding of titanium matrix material, layer to layer within the stack, an interfacial reaction can occur between the fibers and the matrix, resulting in the formation of a layer of intermetallic compound. Fracture events within the plurality of brittle layers of intermetallic compound which occur throughout the laminate have limited the strain capability and thus the strength of previously available titanium composite materials.
Titanium-aluminum alloys containing about 10 to 50 atomic percent Al and about 80 to 50 atomic percent Ti in addition to other alloying elements have been recognized for some time. These alloys are ordered and divided into two major groups: the .alpha..sub.2 alloys are based on the intermetallic compound Ti.sub.3 Al, and the alloys based on the intermetallic compound TiAl. Both groups are referred to as titanium aluminides and have good high temperature strength and oxidation and creep resistance, but are relatively brittle and hard to handle at room temperature.
Fiber-reinforced titanium aluminide matrix alloy composites, in which the alloy contains more than about 10 at .% Al, are currently used only on an experimental basis. Attempts to roll these alloys into sheetstock thinner than about 0.5 mm have provided little success. Consequently, researchers wishing to employ these alloys in fabricating composite materials have had to resort to chemical milling or grinding of sheetstock in order to provide foil of desired thickness, typically about 0.1 to 0.3 mm, thereby greatly increasing material cost.
The high temperature resistance of titanium-aluminide base alloys containing more than about 10 atomic percent Al requires higher composite consolidation and bonding temperatures. Such higher temperatures generally increase the interfacial reactions between the composite reinforcing fibers and the alloy matrix. What is desired is a method for producing composite sructures from titanium-aluminide base alloys containing more than 10 atomic percent A1 wherein the reaction zone, i.e., the region of reaction at the interface between the matrix and the fiber, is minimized, if not eliminated.
It is therefore an object of the present invention to provide an improved method for fabricating composite structures using a titanium-aluminum alloy containing at least about 10 atomic percent Al.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description.